Move purge logic for transient changes below the NodeBuilder interface

Currently transient changes are purged down to a persisted branch once a certain size threshold is hit. This logic currently resides in RootImpl. However this approach doesn't catch the case where a lot of content is added by a commit hook (e.g. index updates) since commit hooks operate on the NodeBuilder level which is below Root.

The aim of this issue is to pursue ways to push the purge logic further down below the NodeBuilder interface.